Re: UC3843 power supply help needed!!!


colin wrote:
> <seegoon99@xxxxxxxxx> wrote in message
> news:1116227614.747750.258110@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Hi to all.
> I have built a power supply using a uc3843 chip and need some help.
> The supply takes 220Vac and steps it down to 14Vdc(approx).
> I have managed to get the supply working OK , but I still need a bit
of
> help.The schematic is attached , in LTSpice format.I modeled the
> schematic and it seems to be ok , but I'm not sure how much you can
> trust a simulation it this sort of level.
> I've attached the model file for anyone who would like to simulate
the
> schematic. Hope it does not make the file to big , but I don't have
any
> access to any other binary groups to send it to.
>
> When I first powered up the circuit the output was very noisy , and
got
> worse as the load increaced.I found by lots of trial and error that
if
> I put a small(100pF) cap across R11(3k3) the output stabilizes and
> everything seems good.There is a bit of high frequency noise on the
> output at the switching frequency , but I am sure this is to be
> expected.
> Now the problem...
>
> I found by checking the drive to the FET that the loop seems unstable
> at certain loads.As I increace the load from 0A to 2.5A
> you can see the gate drive pulse width increacing as expected , but
at
> certain loads it seems to jump all over the place and the scope can't
> trigger.From about 300mA to say 1,5A the gate drive is a mess , and
> after that it seems to settle down nicely.
>
> How do I go about fixing this.It does not seem to be causing a
problem
> on the output , but I would like to fix it up anyway.
> I am sure it is a loop/compensation problem , but I am not sure how
to
> go about fixing it.I have tried changing the gain resistor(R5) to
100k
> and 147K to no avail.Same with the compensation cap C2(100p / 200p).
I
> have also fiddled with the resistor/cap combination around the TL431
,
> also without much luck.
> I am not very experienced with this type of power supply , so I am
in
> the dark a bit!!
> It may also be a board layout problem.I did the board in a hurry
> and did not pay HUGE amount of attention to layout , but I did try
and
> keep high current areas away from control lines etc.It is single
sided.
> This is a sort of hobby project , so time is not really of the
> esscence. I only have a scope and meter to work with , so any
solutions
> that involve using other expensive equipment will be out of my
> capability.My maths is also not very hot , so any solution that
involve
> lots of complex maths may be beyound me , but post them anyway as the
> may be of interest to someone else.(they will be of interest to me ,
> even if over my head!!)
>
>
> ive used this chip in probaly a dozen designs, its a realy neat
little
> package, i cldnt load your circuit, but if u cld post a jpg il have a
look
> at it, the spec *** for this device is realy good, if u have loop
> stability problems you realy need to analyse your loop
charecteristics
> carefully.
>
> with opto feed back you need to acount for the wide variation in loop
gain
> they can have.
>
> some designs rely on a smal amount of esr in the output caps to
ensure
> stability, although persoanly i would try to avoid this.
>
> with current mode control its realy easy to charecterise the output,
its
> just like a big integrator, so
> any roll of in the rest of feedback loop needs to occur when the
overall
> gain is less than 1. finding the loop gain at wich it becomes
unstable and
> reducing it by a factor of 2 or so is one posibility.
>
> Colin =^.^=


Hi there. I don't have any access to any other groups , how can I post
a jpg to this group?
Do you have an email I can send it to.
Cheers
Rob

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